Aspergillus flavipes as a novel biostimulant for rooting-enhancement of Eucalyptus

In recent years, forest breeding programs have increased Eucalyptus production for commercial purposes; however, high-performing Eucalyptus clones have had problems with propagation, especially when rooting. Nevertheless, studies have shown that inoculation of microorganisms producing indole-3-acetic acid (IAA) is an especially productive procedure to help these clones breed. In this study, we evaluated the production of IAA and analogues in 16 microbial strains. For the first time, a high IAA production was described in the strain Aspergillus flavipes (ATCC® 16814™), and A. flavipes was shown by LC-MS/MS to produce IAA through a tryptophan-dependent biosynthetic pathway. A. flavipes reached the highest IAA production when cultivated under solid-state fermentation in an optimized medium composed of soybean bran, water and tryptophan. We mixed the fermentation products in solid form (SF) and liquid form (LF) with the substrate Carolina I® and then planted the cuttings of the hybrid Eucalyptus grandis x E. urophylla (clone IPB2). In fact, treatments with 40–120 mg kg−1 of SF increased the adventitious rooting rate, root length and both root fresh and dry mass, while 120 mg kg−1 of LF increased root length and dry mass. Additionally, there was no toxicity on fibroblasts (NIH/3t3), and, therefore, the plant biostimulant was confirmed as a novel, non-toxic, and eco-friendly solution.